Flexography is a method of printing that is commonly used for high-volume runs. Flexography is employed for printing on a variety of substrates such as paper, paperboard stock, corrugated board, films, foils and laminates. Newspapers and grocery bags are prominent examples. Coarse surfaces and stretch films can be economically printed only be means of flexography. Flexographic printing plates are relief plates with image elements raised above open areas. Such plates offer a number of advantages to the printer, based chiefly on their durability and the ease with which they can be made.
Although photopolymer printing elements are typically used in “flat” sheet form, there are particular applications and advantages to using the printing element in a continuous cylindrical form, as a Continuous In-The-Round (CITR) photopolymer sleeve. CITR photopolymer sleeves add the benefits of digital imaging, accurate registration, fast mounting, and no plate lift to the flexo printing process. CITR sleeves have applications in the flexographic printing of continuous designs such as in wallpaper, decoration and gift-wrapping paper, and other continuous designs such as tablecloths, etc. Thus, CITR sleeves enable flexo printing to be more competitive with gravure and offset on print quality.
A typical flexographic printing plate as delivered by its manufacturer, is a multilayered article made of, in order, a backing or support layer, one or more unexposed photocurable layers, a protective layer or slip film, and a cover sheet. A typical CITR photopolymer sleeve generally comprises a sleeve carrier (support layer) and at least one unexposed photocurable layer on top of the support layer.
The backing layer lends support to the printing element. It is typically a plastic film or sheet about 5 mils or so thick, and may be transparent or opaque. Polyester films, such as polyethylene terephthalate film, can suitably be used as the backing. In the case of printing sleeves, metals, such as nickel and steel, or polymer/fiber composite materials, may also be usable as the backing layer.
When only a single photocurable layer is present, it may be anywhere from about 25 to about 275 mils thick, and can be formulated from any of a wide variety of known photopolymers, initiators, reactive diluents, etc. In some printing elements, there is a second photocurable layer (referred to as an “overcoat” or “printing” layer) atop this first, base layer of photocurable material. This second layer usually has a similar composition to the first layer, but is generally much thinner, being on the order of less than about 10 mils thick. The slip film is a thin (approximately 0.1–1.0 mils) sheet, which is transparent to UV light, and which protects the photopolymer from dust and increases its ease of handling. The cover sheet is a heavy, protective layer, typically polyester, plastic, or paper.
Typical prior art methods for making flexographic printing plates may be found, for example, in U.S. Pat. Nos. 4,045,231, 5,223,375 and 5,925,500, the teachings of which are incorporated by reference herein in their entirety. Typical prior art methods for making printing sleeves may be found, for example, in U.S. Pat. Nos. 4,871,650, 5,798,019, 5,916,403, and 6,424,327, the teachings of which are herein incorporated by reference in their entirety.
It is highly desirable in the flexographic prepress printing industry to eliminate the need for chemical processing of printing elements in developing relief images, in order to go from plate to press more quickly. An early attempt to reduce solvents, and the inherently longer drying required for solvent developing was the aqueous developable flexographic printing plate, as taught in U.S. Pat. Nos. 4,177,074, 4,517,279, 5,364,741 and 6,017,679, the teachings of which are herein incorporated by reference in their entirety. However, the use of water to develop relief is still a “processing” step. In addition, water-developable printing plates have inherent disadvantages, such as limited print performance and the generation of wastewater.
Thermal mass transfer plates, such as DuPont Cyrel® FAST™, are gaining popularity because they are chemical free. In the case of the FAST™ approach, the thermal process of removing the uncured non-image areas of the photopolymer is carried out after cross-linking the image areas of the plate. This approach is demonstrated in U.S. Pat. No. 6,171,758, and in Patent Nos. WO 0118604 and WO 0188615, the teachings of which are herein incorporated by reference in their entirety. Since the photopolymer is “dense”, removing of the uncured non-image areas takes a substantial amount of time to achieve. Customers must also invest in a special and additional proprietary thermal processor.
Laser-engraving systems from Fulflex and BASF (called LEP) are also process-free. An example of this technology is found in Patent No. EP 0 982 124 A2, the teachings of which are herein incorporated by reference in their entirety. In the BASF and ZED/Fulflex approach, the photopolymer/rubber is cured or cross-linked prior to the engraving step. Once again, because of the high density of these materials, the thermal engraving step is long and tedious. Additionally, high resolution is difficult to achieve. Thus, the disadvantage of prior art engraved plates is a combination of limited resolution and throughput.
Directly engraving a relief-printing element with a laser is a highly desirable concept. However, CO2 engraving lasers lack beam resolution and cause anomalies due to heat dissipation. The resolution of such systems is limited to well below 133 lines per inch (LPI) on a practical basis. Infrared (IR) lasers such as Nd-YAG lasers are extremely high in resolution and are precisely controlled. However, these lasers lack the necessary power and reactivity to engrave conventional photopolymers and may be too slow due to mass transfer limitations in dense “cured” photopolymer or rubber systems.
Digitally imaged continuous photopolymer sleeves add the benefits of digital imaging, accurate registration, fast mounting, and no plate lift to the flexo printing process. In addition, digital sleeve printing with CITR enables converters to save money on a total cost basis when compared to flat flexo plates due to the elimination of plate mounting in addition to significant efficiency gains once on press. Furthermore, such continuous printing elements can be well suited for mounting on laser exposure plate-setters.
CITR photopolymer sleeve products with limited performance have been available to the market for many years. Trade shops, using mostly traditional photopolymer and sleeve raw materials in a complex manufacturing process with long lead times, currently produce these sleeves. The formation of “seamless,” continuous photopolymer sleeve has traditionally been accomplished by a process called “Seamex”. The Seamex process involves wrapping a layer of flat sheet elements, called “sleeve gum” (for example KOR® SG supplied by MacDermid Printing Solutions), to a nickel sleeve having a heat-activated primer coat to bond with the material so that the ends of the plate are joined together. The entire assembly is placed in an oven to cure and bond the photopolymer layer to the primer coat and melt the ends of the layer together. The photopolymer layer on the sleeve is ground to the necessary thickness, and then spray coated or ring coated with an IR sensitive layer. The process of wrapping, curing, melting, grinding and spraying the photopolymer layer to the sleeve can take about 1.5 to 2 days to accomplish. The cylindrical sleeve is then imaged on typical plate-setters, and the image set by exposure to UV light, and processed to wash away the unexposed areas of the layer on the cylinder, forming a relief surface for printing. These processes for joining the edges of a plate into a cylindrical form have been disclosed, for example, in U.S. Pat. No. 4,758,500, Great Britain Patent No. GB 1,579,817, German Patent No. DE 28 44 426, and European Patent No. EP 0 280 103, the subject matter of each of which is herein incorporated by reference in its entirety.
A problem with the prior methods of joining the edges to form a continuous cylinder is that sometimes the joined seam is visible in and interrupts the printed image. Moreover, image degradation may occur due to repeated exposure to heat, first in the extrusion and then during the “heat curing” process. It would be desirable to have a processless digitally imaged continuous sleeve option in the market, having the advantages of processless digital imaging described above and usable with CITR photopolymer sleeves.
A solution to the problems seen in prior art printing plates and printing sleeves may lie in the use of a curable thermoplastic elastomer that contains micro-bubbles to form a “processless” printing element. Preferably the elastomer is radiation-curable and the radiation source is selected from UV lights and Electron Beam (EB) sources. The inventor has surprisingly discovered that EB curing may be superior to UV curing for this application for reasons discussed in more detail below The use of EB curing is well-known in the prior art as evidenced by the teachings in U.S. patent application Ser. No. 2003/0054153 A1, EP A 84-18 0107608, EP A 86-25 0184598, EP B1 00852596, EP B 02-19 0726290, and U.S. Pat. No. 6,124,370, the teachings of each of which is herein incorporated by reference in its entirety.
The curable composition is essentially a photocurable elastomeric uncured foam that is laden with a material that absorbs light at a selected wavelength. In a preferred embodiment, this material is a dye (or pigment) that is both IR absorbing and UV transmissive. As the IR laser strikes the material that absorbs light at a selected wavelength (i.e., dye), it transfers IR energy into heat, causing “laser collapse” of the micro-bubbles or microspheres. Because the photocurable elastomeric material consists of foam cells which are only microns in size, the ablation-to-depth process can occur much more quickly, using much lower energy than is required in true mass transfer systems such as mask ablation or polymer engraving. In addition, the lower density and the corresponding lower heat energies involved in this process act to prevent conductance of heat energy to adjoining cells, thus limiting thermal damage and having the potential for higher resolution than traditional laser engraving. After all of the non-printing (relief) areas have been laser collapsed, there may be an additional process step to laser collapse the top layer to form a denser printing surface. This denser printing surface can also be created by a “bump” laser-exposure in concert with the regular exposure. A “bump” or “flash” exposure refers to a quick exposure, generally of less than about 1 micro-second. The photopolymer is then flood exposed, preferably using UV- or EB-curing, to cross-link the formed image for enhanced physical properties. Finally, the process may contain a conventional detacking step.
The advantage of this “low density” approach is that it may be usable in any of the conventional plate-setters in the industry, with only a change in the software that is used to control the energy density; no major investment in hardware is needed. The disadvantage of UV-imaging through a “foam” is obviated because the imaging is done by the interaction of the IR laser with the microspheres. Curing is used simply to set the image in place. Furthermore, one avoids the washout process step, and hence has the workflow advantage of going from plate to press much more quickly than in conventional flexographic printing elements, while at the same time reducing solid waste generation.
U.S. Pat. No. 6,159,659 and U.S. Pat. No. 6,090,529, both to Gelbart, the teachings of which are incorporated herein by reference in their entirety, disclose methods for directly creating a raised image on a flexographic printing surface. These patents disclose laser ablation of an intermediate layer that comprises an elastomer and a high concentration of plastic or glass microballoons, in order to form recessed areas on the surface. In addition, these patents disclose controlling the intensity of a laser beam and the dwell time of the laser beam in each spot so that the laser power applied to each part of the surface is sufficient to cause localized melting of the intermediate layer. The dwell time is sufficiently long so as to produce viscous flow of the melted material, while the laser intensity is insufficient to cause complete ablation of the intermediate layer. In one example, the printing plate is made from a closed-cell black polyurethane foam, where the foam has a density of about 10% that of solid polyurethane. U.S. Pat. No. 6,159,659 further discloses that when the plate is “cut” or ablated, with a laser at the operating wavelength, the cutting action is self-limiting because of the insensitivity of the backing at the operating wavelength, which avoids damage to the backing.
The present invention comprises a collapsible photosensitive elastomer composition comprising a curable elastomer, a material that absorbs laser light at a selected wavelength, and microspheres used for making digitally imageable relief-printing elements. In contrast, the U.S. Pat. No. 6,090,529 patent and the U.S. Pat. No. 6,159,659 patent do not disclose a photosensitive elastomer and do not disclose crosslinking the composition of the formed image in order to enhance the physical properties of the printing element. In addition, the U.S. Pat. No. 6,090,529 patent and the U.S. Pat. No. 6,159,659 patent do not disclose how the foam intermediate layer behaves as a printing plate. The patents disclose a pigment/dye that is carbon-based, which is not suitable for use in the present invention because it would interfere with the photocurable aspects of the invention.
The present invention also describes a cross-linkable raw material that is cured after laser imaging for added physical strength necessary for press life durability. The present invention advocates the use of microspheres, to yield excellent image fidelity and consistency. As explained in more detail below, the choice of the microspheres and the material that absorbs laser light at a selected wavelength are key elements to the success of this invention.
The new concept of the present invention addresses the market need for eliminating the need for chemical processing of printing elements, by using a very low-density photopolymer printing element that is impregnated with infrared (IR) sensitive micro-bubbles, that collapse when irradiated with a laser. Subsequently the photopolymer can be cured to cross-link the material to enhance its physical properties.